Silicon wafers for use in the semiconductor industry are produced by a variety of methods. One such method is the Czochralski or "CZ" method. In the CZ method a silicon seed crystal of known orientation is immersed in a molten pool of silicon. The seed crystal triggers solidification of the silicon as a single crystal. As the crystal is mechanically pulled upward from the pool, the molten silicon solidifies to form an ingot. Typically, argon gas flows downwardly through the CZ apparatus to cool the solidifying crystal and to blow impurities away from the molten silicon. Silicon wafers can be manufactured from the solid ingot by machining and polishing.
A specifically constructed apparatus is used to accurately control the various parameters needed to ensure that high-quality crystals are produced. The CZ apparatus includes a crucible to hold the molten silicon. Such crucibles are typically made of fused silica (quartz). Fused silica has a high melting point and thermal stability, and is relatively non-reactive with molten silicon. However, during higher temperature melt-in periods the fused silica crucible becomes soft. Near the melting point of silicon, the crucible becomes so soft that it requires the support of a heat resistant and rigid outer crucible ("susceptor"). The susceptor and other components in a CZ apparatus that are subjected to high temperatures are typically fashioned from graphite because of its high temperature capability, relative chemical inertness, and its cleanliness properties, specifically freedom from metallic impurities. Other CZ apparatus components made from graphite include, e.g., pedestals upon which the susceptors sit, heater elements, spill trays, gas flow components, and heat shields.
The contact between the quartz crucible and the graphite components, during the crystal growing process, limits the lifetime of the graphite components. That is, at temperatures around the melting point of silicon (1410.degree. C.), silicon oxide (SiO) gas evolves from the quartz crucible. The SiO gas causes chemical reactions to occur with the graphite susceptor and other graphite or carbon components in the CZ apparatus that are in contact with the gas. Oxygen from the SiO gas erodes the graphite, forming, inter alia, carbon monoxide (CO) and carbon dioxide (CO.sub.2). CZ components used in producing other crystals, such as compound semiconductors (e.g., GaAs), typically do not encounter this erosion reaction because of the absence of silicon and oxygen-containing gases within the apparatus.
Silicon, on the other hand, combines with graphite to form silicon carbide (SiC). In certain regions of the graphite susceptor, the reaction whereby Si and graphite convert to SiC predominates, and a durable SiC surface is naturally formed by the process. In other regions, due to a variety of factors, such as the direction and velocity of gas flow in the CZ apparatus, a graphite-erosion reaction is favored. As the graphite components erode, they eventually cease to be useful and must be replaced. Particularly, erosion of the graphite typically thins the walls and floor of the susceptor, weakening those areas to the point whereby failure by cracking may occur. Additionally, such erosion may cause a void or cavity to be formed in the susceptor. Such a void or cavity may allow the quartz crucible to bulge into the susceptor to the extent that the crucible may rupture. Failure of the susceptor and/or the crucible poses a significant safety hazard. Particularly, spillage of the molten silicon through the apparatus may result in equipment damage or serious injury to the operators of the apparatus. Due to these detrimental and costly effects of erosion of the graphite susceptor and other carbon CZ components, the components, particularly the susceptors, are generally replaced after a limited period of use rather than risk the deleterious effects of erosion.
Coatings have been applied to the surface of the graphite components. For example, in U.S. Pat. No. 5,476,679 (Lewis et al.) a method for forming a glassy carbon coating on a susceptor is disclosed. The coating is formed to prevent contamination of the molten silicon by metallic impurities that may be contained within the graphite susceptor. Although this glassy carbon coating adequately blocks impurities contained within the susceptor from entering the crystal growing environment, it has been found that such a carbon coating does not fully protect susceptors against the effects of the erosion and conversion reactions just described. Moreover, the method disclosed in the Lewis et al., patent does not allow a protective coating to be formed on only the portions of the susceptor most likely to suffer from erosion but, rather, requires the entire susceptor be coated, thus requiring more coating material than would be necessary to protect the susceptor from erosion. Furthermore, parts that are fully coated are not easily evacuated and undergo a slow out-gassing behavior during the operation. Such behavior causes "virtual leaks" that behave like actual vacuum leaks and cause a loss in production time.
Silicon-carbide coatings have been applied to graphite components using conventional chemical vapor deposition (CVD) techniques. This method has been very successful for applications such as crystal epitaxy where temperatures are usually below 1000.degree. C. For CZ susceptors, such CVD coatings tend to crack or delaminate after limited use. One explanation for this may be that the deposition temperature for the CVD coating is usually lower than the operating temperature of the CZ apparatus. Due to a difference in the coefficient of thermal expansion between graphite and silicon carbide, at CZ operating temperatures a tensile stress is created within the coating and a sheer stress is created between the graphite and the coating. This results in cracking and delamination of the coating from the carbon component when heated. Moreover, CVD is expensive, and partial coating of components using CVD would require expensive and time consuming masking procedures.
Accordingly, there is a need for protective coatings on graphite components wherein such coatings are durable, inert, highly resistant to heat, and may be applied or formed in a relatively inexpensive and simple manner on all or part of the component surface.